LAUSR

Lanthanide-doped sodium yttrium fluoride (NaYF4) nanocrystals have been heavily studied for their upconverting properties [1–3] and specifically their potential for laser cooling [4,5]. NaYF4 is also notable for its two crystallographic polymorphs, cubic (α) and hexagonal (β). While there has been much research into synthetic methods of making one or the other [6–8], the actual, heat-driven phase conversion mechanism between the two remains poorly understood [9]. In 2010, Shan and colleagues reported their observation that α-NaYF4 nanoparticles form hollow structures upon heating in a process analogous to the Kirkendall effect, which they propose is a step in the phase transformation [10]. Here we report an observation of that void formation using in situ heating transmission electron microscopy (TEM).